Make Data Source available for the whole project, instead of a test case.

That would be a good idea, but nevertheless the DataSource should also be usable on TestCase level only. 

So if possible, make this optional for both?

It's been awhile. Any update from SoapUI Product Owner or BAs if there is a chance that we'll see this become a reality? :-)

May be it would make a better case if the use case(s) is explained with good and generic examples.

Can't imagine use of the request without details.

It would be great to get some use-case examples

I don't know what the OP needs. But, I can guess one sample use case.

Run all tests in project for USA, Europe region. For certain test cases, run the tests for China & India *also*. Think of this as inheritance in data source.

PS - If these kind of things and customizability are a huge deal for the OP, then it might be time to consider using a programming language instead of ReadyAPI. That way, you have the freedom to implement whatever you like, provided you can bear the costs.